In industry, there are many continuous separation processes based on selective adsorption of at least one component among several in a mixture of fluids, notably processes known as simulated countercurrent chromatography processes where the property of certain porous solids, in the presence of liquid, gaseous or supercritical mixtures, of retaining more or less significantly the various constituents of the mixture is used.
Separation or fractionation processes based on chromatography are most often implemented in a device comprising a set of columns or column fractions interconnected in series, forming a closed loop. Injection points for the mixture to be separated and the solvent or desorbent, and fluid extraction points: extract and raffinate, delimiting different zones are distributed along this loop. Identical liquid flows run through all the columns or column fractions of the same zone.
The devices most often consist of four main zones. However, some have only three zones (in this case, the zone contained between the raffinate draw-off point and the solvent injection point is omitted). There are also devices with five zones where part of the extract separated from the solvent is reinjected between the extract draw-off point and the feedstock injection point. Others may comprise five to seven zones where secondary fluids allow to flush lines carrying successively several fluids, so as to prevent contamination.
A porous solid of determined grain size forms the stationary phase. The mixture to be separated is fed into the column, then displaced by a carrier fluid or desorbent, and the various constituents flow out successively according to whether they are retained more or less significantly by the stationary phase.
In a real countercurrent process (FIG. 1), a fixed and constant concentration profile develops in a separation column 1 where the position of the points of injection of a feedstock A+B, of an eluent S, and of draw-off of an extract EA and of a raffinate RB remains fixed. The adsorbent solid 3 and the liquid 2 circulate in a countercurrent flow. A solid carrying system and a recycling pump P, both placed on the location of the column (at the junction of zones I and IV) where the only species present in the liquid as well as in the solid is the elution carrier fluid, allow respectively to run the solid from the bottom to the top and conversely the liquid from the top to the bottom.
Processes known as simulated moving bed processes allow to avoid a major difficulty inherent in real moving bed processes, which consists in correctly circulating the solid phase without creating attrition and without considerably increasing the bed porosity in relation to that of a fixed bed. In order to simulate the displacement thereof, the solid is placed in a certain number n of fixed beds (generally 4.ltoreq.n.ltoreq.24) placed in series and it is the concentration profile which is displaced at a substantially uniform speed all around a closed loop.
In practice, successive shifting of the injection and draw-off points is performed by means of a rotary valve or more simply by means of a set of properly controlled on-off valves. This circular shift, performed at each period, of the various incoming-outgoing flows in a given direction amounts to simulating a displacement of the solid adsorbent in the opposite direction.
Countercurrent or cocurrent simulated moving bed chromatography processes are for example described in U.S. Pat. No. 2,985,589 or U.S. Pat. No. 4,402,832.
A system for separating, in the presence of at least one eluent, a feedstock comprising at least two constituents into at least two fractions generally comprises n closed-loop chromatographic columns or column sections mounted in series (generally 4.ltoreq.n.ltoreq.24) where a pressurized liquid, supercritical or gaseous mixture is circulated, the loop comprising at least one feedstock injection current, at least one eluent injection current, at least one extract draw-off current and at least one raffinate draw-off current, the constituent preferably sought being mainly either in the extract or in the raffinate.
The main inlet flow rates are the feedstock flow rate and the eluent flow rate. The outlet flow rate is the extract flow rate. The raffinate is withdrawn under pressure control. The raffinate flow rate is equal to the sum of the inlet flow rates minus the extract flow rate. In addition to these controlled flow rates there is a controlled recycle flow rate whose value also depends on the position of the pump at a given time. The relative location of each of the four flows around the beds is calculated so as to obtain a satisfactory behaviour depending on the type of separation to be performed and thus defines four distinct zones in the case of the process shown in FIG. 1.
U.S. Pat. Nos. 5,457,260 and 5,470,482 describe a process controlling a simulated moving bed system for separating a mixture of constituents, comprising two loop interconnected multiple-bed columns, where at least one characteristic such as the purity of a constituent or the yield thereof or a combination of both is controlled. The process comprises measuring the concentration of the various constituents of the mixture circulating in the interconnection circuits of the columns, notably by near-infrared spectroscopy, and using an iterative adjustment algorithm of multivariable regression type or of neural network type which tends to decrease the difference between the actual value of the characteristic and a set value up to a certain threshold. The algorithm used is of the "black box" type with all the drawbacks linked with this type of approach: a considerable implementation time since the result is obtained only after many tests, it is exploitable only in the field of the tests carried out and it is obtained with very little precision, that of the modeling of nonlinearities in general.
In the text hereafter, the following terms designate:
controlled variables: variables which must be constantly close to a precedingly specified set value and which show the smooth running of the process. It may be, for example, the purity of the constituents of an extract, the yield of the separation unit for a given constituent, etc.:
operating variables: variables which can be modified by the operator, such as flow rates or valve switching periods allowing to simulate the displacement of the beds, etc.;
control variables: variables which mainly act on a single zone, for example on the part of the concentration profile contained in a zone. These control variables are determined by the control algorithm and are translated into operating variables.
It may be reminded that the aim of an advanced process intended for control of the running of a separation loop is to calculate a control law (all of the values of the operating variables in the course of time) in order to:
control operation, i.e. to calculate a control law capable of making the transition between two distinct values of one or more controlled variables selected a priori, and
regulate operation, i.e. to calculate a control law capable of compensating as much as possible (in advance or at least asymptotically) for all the external disturbances acting on the process so that the controlled variables selected a priori keep a quasi-constant value.
In the case of a simulated moving bed unit, regulation can also compensate for disturbances due to an evolution in time of the thermodynamic and geometric parameters of the adsorbent (of course for a limited deterioration of the properties of the adsorbent).
These objectives are fulfilled with the process intended for automatic control of a process for separating constituents of a mixture of circulating fluids according to the invention, which allows to overcome the aforementioned drawbacks. It is not based on a "black box" type technique but on a more controlled approach permitted by a nonlinear modeling of the separation process.